Isolation valve

ABSTRACT

A valve for isolating the interior of a glow discharge chamber from the atmosphere and other reactants. The valve includes a body mounted in an aperture of the chamber. A slit-like opening is provided therein to allow the interior of the chamber to communicate with interrelated elements of a deposition system. A source is provided for introducing inert gas into the opening under pressure somewhat greater than that of reaction gases and plasma within the chamber to create an effective, non-reactive gas curtain.

CROSS REFERENCE

This is a continuation of application Ser. No. 444,923 filed Nov. 26,1982, now abandoned, which is a continuation of application Ser. No.244,385 filed Mar. 16, 1981 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and systems which may beutilized to mass-produce semiconductor devices. In particular, thisinvention pertains to the production of amorphous semiconductor devicesby continuous, as opposed to batch, processing.

2. Description of the Prior Art

Crystalline materials which feature a regular lattice structure wereformerly considered essential in the manufacture of reliablesemiconductor devices. While solar cells, switches and the like havingfavorable characteristics continue to be so manufactured, it isrecognized that preparation of crystalline materials introducessubstantial costs into the semiconductor industry. Single crystalsilicon and the like must be produced by expensive and time-consumingmethods. Czochralski and like crystal growth techniques involve thegrowth of an ingot which must then be sliced into wafers and are thusinherently batch processing concepts.

Recent developments in the field of devices formed of amorphoussemiconductor materials offer a potentially significant reduction inproduction costs. In particular, solar cell technology, which isdependent upon the production of a large number of devices to comprise apanel, is critically affected by processing economies. The feasibilityof semiconductor devices produced by amorphous, as opposed tocrystalline, materials is disclosed, for example, in U.S. Pat. No.4,217,374 of Ovshinsky and Izu for "Amorphous Semiconductors Equivalentto Crystalline Semiconductors". A silicon solar cell produced bysuccessive glow discharge deposition of layers of various conductivitiesand dopings and its process of manufacture are described in UnitedStates patent application Ser. No. 887,353 of Ovshinsky and Madan filedMar. 16, 1978 for "Amorphous Semiconductors Equivale to CrystallineSemiconductors", now U.S. Pat. No. 4,226,898. Both of these prior artpatents are hereby incorporated by reference as representative ofamorphous semiconductor technology.

The feasibility of amorphous devices becomes apparent in light of thedrawbacks inherent in production of crystalline devices. In addition tothe aforementioned inherently "batch" nature of crystal growth, asubstantial amount of the carefully grown material is lost in the sawingof the ingot into a plurality of useable wafers. Substantial surfacefinishing and processing effort is often required thereafter.

Generally, the production of amorphous devices utilizes batch methods.As in the case of crystalline devices, such production methods impairthe economic feasibility of amorphous devices such as solar cells byintroducing "dead time" during which valuable equipment sits idle.Recently, efforts have been directed to the possibility of producingamorphous semiconductor devices by continuous processes. United Statespatent application of Izu, Cannella and Ovshinsky for "Continuous SolarCell Production System" (to be filed) the property of the assigneeherein, discloses a system and method for the continuous production ofsolar cells of amorphous material. In that application, there isgenerally disclosed a system for advancing a weblike substrate ofmaterial through a plurality of modules to produce a plurality of solarcells.

SUMMARY OF THE INVENTION

The present invention supplies a necessary element for the realizationof continuous systems for the production of amorphous semiconductordevices by providing a valve to isolate the interior of a glow dischargechamber from the atmosphere and other reactants. A pair of such valvesis mounted at inlet and outlet apertures provided in the chamber toallow a substrate to advance therethrough in continuous fashion. Eachvalve includes a slit-like opening for the web-like substrate. Means areprovided for maintaining inert gas within the opening at a pressuresomewhat greater than that of the reaction gas and plasma within thechamber to create a curtain of non-reactive gas.

Other advantages and features of the present invention will becomeapparent from the following detailed description wherein like numeralscorrespond to like features throughout:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are side and top sectional views of a continuous layerdeposition system according to the present invention;

FIGS. 2A through 2C are front, cross-sectional and top viewsrespectively of a slit valve according to the invention for applying anisolation curtain of inert gas at the entrance and exit of each chamber;

FIGS. 3A and 3B are side and cross-sectional views of a depositionchamber according to the present invention; and,

FIGS. 4A and 4B are top and cross-sectional views of a cathode accordingto the present invention while FIGS. 4C through 4E are plan views of theinternal baffle plates of the cathode and FIG. 4F is an alternativeembodiment of the cathode.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1A is a side sectional view of asystem incorporating the present invention. In a sense, it discloses an"assembly line" for the application of layers of amorphous semiconductormaterial to a flexible film substrate 10. The substrate 10, shown inedge view throughout FIG. 1A, comprises a continuous sheet of web-likematerial, preferably about one to four feet in width. As the inventionis directed in effect to the deposition of a number of layers ofamorphous semiconductor material, it represents an intermediate step inthe processing of complete semiconductor devices. A complete device mayadditionally include other elements, the application of which requiresparticularized processing. Such processing may occur both prior to theplacement of the substrate 10 (as a roll) on a reel 12 of a feed section14 and subsequent to being gathered into a roll (a plurality of layersof amorphous semiconductor material having been deposited thereon) on areel 16 of a takeup section 18.

In the production of solar cells in accordance with the presentinvention, a flexible metallic foil or metallic surfaced foil isutilized as the substrate 10. Certain device configurations may requirea thin oxide insulating layer and/or a series of base contacts prior tothe application of the layers of amorphous material. Thus, for thepurpose of this description, the term "substrate" will be understood toinclude not only a flexible film, but also elements added thereto bysuch preliminary processing. Both preliminary and finishing steps in theprocessing of solar cells are disclosed in the patent application ofIzu, Cannella and Ovshinsky.

The substrate 10, wound through the system of FIG. 1A from the feed reel12 to the takeup reel 16 in a generally counterclockwise direction, isadditionally guided by means of intermediate idler reels 20, 22, 24 and26. In traversing the path they define, the substrate 10 passes throughchambers 28, 30 and 32 wherein the closely regulated deposition ofP-type, intrinsic, and N-type layers of amorphous semiconductor materialtakes place. Although the preferred embodiment of the system asillustrated contemplates the continuous production of PIN solar cells ofamorphous silicon and the discussion will proceed upon such basis, itwill be appreciated that changes of such variables as dopants and otherconstituents of the reaction gas, the sequence of chambers between thefeed section 14 and the takeup section 16, and the like will result in acorresponding rearrangement of the cross section and effect redesign ofthe device produced. Thus, various devices can be produced in acontinuous manner by the present invention wherein other constituentsand deposition sequences are employed and such are considered within itsscope.

The intrinsic (material) deposition chamber 30 is considerably longerthan either the P-type layer deposition chamber 28 or the N-type layerdeposition chamber 32. The continuous deposition processes thereinrequire that the substrate 10 travel from reel 12 to reel 16 atsubstantially constant speed. As N-type, intrinsic and P-type layers arecontinuously deposited on various portions of the travelling substrate10, the layer thickness, which is a function of the time spent within agiven chamber serves, in large measure, to determine the relativelengths of the chambers.

The exemplary device produced by the system of FIG. 1A generally willproduce thickness ratios in the range of 10 to 30:1 between theintrinsic and doped layers which occasions an intrinsic chamber of aboutsixteen feet in length and P- and N-type chambers of about two andone-half feet in length. Again, the manufacture of other devices havingdiffering relative dimensions will dictate, to a large extent,appropriate chamber lengths.

If masking of the devices is desired, belt-like masks 34, 36 and 38 eachincluding a plurality of strips (for example one inch wide), may exposecorresponding strip-like areas across the width of the substrate 10 asit travels through the chambers 28, 30 and 32. The strips 39 whichcomprise the masks are seen in FIG. 1B, a partially sectioned top viewof the system of FIG. 1A. Each of the masks 34, 36 and 38 circulatesbetween a pair of rollers 34', 36' and 38', respectively. The pairs ofrollers 34', 36' and 38' may be either independently driven or geared tothe mechanisms of the reel-to-reel advance which drives the substrate 10through the system. In either event, appropriate synchronization ismaintained between the masks and the traveling substrate 10 to assureproper alignment of the deposited layers.

An appropriate amount of tension must be maintained within thetravelling substrate 10 for proper registration with the masks 34, 36and 38. The amount of tension, however, must be carefully regulated asit is believed that excessive or insufficient tension may cause improperpositioning or travelling of the web substrate which in turn may causedamage to the substrate 10 or the material deposited thereon. Theservocontrolled motor drive engaged to the feed reel 12 controls thetension throughout the substrate 10. The motor drive 40 engaged to thetakeup reel 16 is continually adjustable to provide an optimum rate oftravel of the substrate.

Close proximity contact is desired between the masks 34, 36 and 38 andthe advancing substrate 10. In a common PIN solar cell of the typefabricated by the system of FIG. 1A, P- and N-type layers of minutethickness assure that leakage (from separation of mask and substrate)around the strips is likely to be of little significance. Themaintenance of relative intimate contact between the substrate 10 andthe mask 36 during intrinsic layer deposition is critical due to therelatively thick intrinsic layer. As seen in FIG. 1A, the deposition ofintrinsic material occurs in the horizontally-oriented chamber 30. Itshould be understood that the horizontal disposition of the chamber 30is not considered critical and that any elevational position desired maybe employed. A natural sag occurs in the substrate 10, which is alsohorizontal when it passes therethrough, resulting in a degree ofdisplacement along its length throughout the chamber 30. It has beenfound that as much as 5/8 of an inch of such sag may occur as it travelsthrough the sixteen foot horizontal chamber 30.

As shown in FIG. 1A, a plurality of paired pinch rollers 42 acts as anedge guide and web-tensioner for the advancing substrate 10. The rollers42 are an optional feature. When utilized, one need not rely upon thenatural sag in the substrate 10 but can more reliably adjust theposition of the substrate and the mask 36 for proper proximity masking.

Each of the chambers 28, 30 and 32, through which the substrate 10 isadvanced, includes a lamp holder 44 for retaining a plurality of quartzinfra red lamps on one side of the substrate 10 and a cathode 46 (forenergizing a plasma) on the other side of the substrate 10. It will beseen that the reaction gases enter each chamber through a cathode whichadditionally includes means for maintaining uniform gas pressure at thesurface of the substrate 10. A separation valve 48 is located at theentrance and exit of each of the deposition chambers. These valves,discussed in greater detail infra, allow the passage of the continuoussubstrate and mask from chamber to chamber without contaminating thereaction gas environment of each chamber. Control means (not shown inFIG. 1A) associated with each of the chambers, will be disclosed withparticular reference to FIGS. 3A and 3B.

FIGS. 2A through 2C are detailed views of one form of a separation valvefor use in and according to the present invention. The valve allows thepassage of the travelling substrate 10 into and out of each of thechambers; at the same time, it provides a curtain of inert gas toisolate the gas environment of each chamber from each other chamber.Particularly, contamination of the intrinsic deposition chamber bydopant gases present in the other chamber is prevented, which isessential to the manufacture of a high efficiency device.

FIG. 2A is a front elevation view of a separation valve. The valve isformed of a pair of matching manifolds 50, 52. A flange 54 encircles themanifolds 50, 52 when joined together to complete the valve. A pluralityof holes 56 therein allows the valve to be bolted, riveted or the liketo an aperture in either the front or back wall of a chamber. A groove58 is machined into the mating surface of the lower manifold 52. Thegroove 58 is centrally positioned to form a centrally-located slit 60 oropening in the valve when the manifolds 50, 52 are joined together. Theslit 60 provides communication between a chamber wall and the rest ofthe system allowing entry and/or exit of the continuously-movingsubstrate 10 (and associated mask if such is employed).

Turning to FIG. 2B, a side elevational view in cross-section taken alongthe line 2B--2B of FIG. 2A, is provided and one can see that the slit 60is flared or bevelled to assist the initial threading of the substratetherethrough. To further protect the substrate from harm or degradationas it passes through the slit 60, the opposed (top and bottom) surfacesthereof are coated with teflon or like lubricant. An appropriateprotective layer of teflon may be provided by spray deposition andsubsequent baking at 600 degrees Centigrade. In the system of FIGS. 1Aand 1B, the substrate 10, upon passage through the slit 60, is orientedwith its active surface facing downward. In this orientation, the valve48 is located so that the advancing substrate web does not touch thegroove 58 at the bottom of the slit 60, but may brush against itsopposed side.

A gas inlet channel 62 provides communication between a source of inertgas 64 and a cylindrical groove 66 which is at least coextensive withthe width of the slit 60. This configuration may be seen most readily inFIG. 2C which is a top plan view of the valve 48. The width of the slit60 is defined by the distance from the dashed line 68 to the dashed line70. The cylindrical groove 66, one-half of which is formed in the uppermanifold 50 and the other half of which is formed in the lower manifold52, communicates at either end with a gas inlet channel 62 and with agas outlet channel 72. As referenced above, the gas inlet channel 62 isin communication with a source of inert gas 64 whereas the gas outletchannel 72 is connected to a vacuum pump 74.

The gas source 64 and the vacuum pump 74 interact to maintain the flowof an inert gas, such as argon, within the cylindrical groove 66 at apressure somewhat greater than the pressure within the depositionchamber. In this manner, a gas curtain is formed in the interior of thevalve to prevent cross contamination of the reaction gases and, moreimportant, the entry of dopant gases into the intrinsic depositionchamber. Such dopant, gases even in minute amounts, can degrade theintrinsic material which would adversely effect cell performance.

While isolation of the interior of the chamber is the prime object ofthe valve 48, a significant amount of inert gas within the chamber caninterfere with the deposition process. For this reason, the gas pressurewithin the cylindrical groove 66 is maintained only slightly above thatof the chamber (about 1 Torr). Impedance to the escape of the inert gasinto the chamber is increased as the length (along the direction oftravel) of the narrow portion of the slit 60 (seen best in FIG. 2B) isincreased. Thus, the design of the cross-section of the slit 60 inaccordance therewith will serve to minimize the escape of the inert gasfrom the groove 66 into the interior of a chamber.

FIG. 3A is a side elevational view partly in cross section of adeposition chamber 76. A controlled environment is maintained at itsinterior for the glow discharge decomposition process and resultantplasma/substrate equilibrium. The chamber is defined by top and bottomwalls 78 and 80 riveted or otherwise joined to front and rear walls 82and 84 and to side walls 86 and 88. All of the walls are preferably of ametal or metallic alloy which is nonreactive with the various gases tobe introduced into the chamber. As the plasma generated by thedecomposition of the reaction gases is confined to the area between thecathode 90 (whose potential is regulated by an RF power source 100 tocreate the required field between the cathode 90 and the groundedsubstrate 10) and the moving substrate 10, essentially no amorphousmaterial is deposited on the walls of the chamber.

Slit separation valves 92 and 94 are located in the front and rear walls82 and 84, respectively, of the chamber 76. The valves, discussed above,allow passage of the substrate 10 and the plurality of strips whichcomprise the offset masking 96, if used, through the chamber 76.

As the substrate 10 passes through the chamber 76, the depositionprocess is closely regulated on a continuous basis. That is, theessential variables of the production process, and the resultingproduct, are continually monitored and fed back to various correctivecontrols. The chamber 76 is idealized in that it contains a fullcomplement of apparatus for controlling the various process parametersincluding film thickness. The chamber 30 of FIG. 1A, for example,includes means for detecting film thickness of the intrinsic amorphoussilicon layer. As previously mentioned, the intrinsic layer isapproximately 10 to 30 times as thick as either of the P-type or N-typedoped layers. Even though the doped layers are relatively thin, thedetection and measurement of the thickness of those layers may also beaccomplished.

The various reaction gases, which should include a compound of siliconsuch as SiF₄ or silane and at least one alterant element such asfluorine or hydrogen which acts to reduce the density of localizedstates in the energy gap to produce a layer of material havingelectrical properties which closely resemble crystalline silicon, areintroduced into the chamber 76 by means of a reaction gas processor 98.The processor 98 includes a supply of appropriate reaction gas and meansfor evacuating the spent or unreacted gases from the interior of thechamber 76. In addition, the processor 98 preferably includes scrubbingmeans and the like for reclamation of the reaction gases.

The reaction gas processor 98 is coupled to the cathode 90. The cathode90, discussed below, includes unique baffling means so that, in additionto providing an equipotential surface for the formation of a uniformplasma induced by an electric field, it provides a uniform flow ofreaction gas across the surface of the substrate and a uniform removalof the spent gases. The uniformity of the gas flows assures thedeposition of the amorphous material having uniform electrical andoptical properties across the surface of the substrate. The substrate 10should be maintained at a fixed potential, as for example, groundpotential throughout the system as disclosed herein to allow theformation of an appropriate electrical field between the substrate andthe cathode 90.

A detector 102 is located in the chamber 76. The detector 102 may beeither an optical pyrometer or a thermocouple supported by a mountingbracket adjacent to the moving substrate. The detector 102 has aresponsivity to radiation of above five microns in wavelength and iscoupled to servocircuitry 104 which controls a plurality of quartz infrared lamps 106 contained within a lamp holder 108. The spacing betweenthe lamps becomes larger as one progresses from the heating region tothe constant temperature region through the chamber 76 from entrance toexit. This pattern accomplishes, in a first approximation, therelatively quick heating of the substrate to a desired temperature suchas about 300 degrees Centigrade upon entrance into the chamber 76 andits subsequent maintenance thereat for optimum deposition upon exposureto the plasma. The infrared detector 102 is chosen for peak sensitivityat a wavelength of about five microns as a result of the fact that suchwavelength is the peak where 300 degrees Centigrade obtains maximumradiation. The servocircuitry 104 utilizes the radiation pattern for 300degrees Centigrade to generate an error signal which it then converts toa current to regulate the intensity of the quartz infra red lamps 106within the lamp holder 108.

FIG. 3B is a cross-sectional view of the chamber 76 taken along line3B--3B of FIG. 3A. One can see that a source of a radiant beam energy110 is located therein. Energy emerging therefrom is projected alongoptical axis 112 to an optical detector 114 after reflection from thesubstrate 10. The optical detector 114 may include focusing optics and amonochrometer in the event that the source 110 radiates white light;alternatively, the source 110 could be a laser and the detector 114 adetector of laser energy.

The combination of the source 110 and the detector 114 is utilized inthe present system to detect the thickness of the layer deposited uponthe substrate 10. Although only one paired source-and-detectorcombination is shown in FIG. 3B, one can see from FIG. 3A that aplurality of detector/source combinations is envisioned in the presentinvention. Each detector 114 is engaged to a thickness control circuit116. The circuit 116 accepts the output of the detector, processes thesignal through appropriate amplifiers, wave analyzers and the like andthen applies the signal to a feedback or servocontrol circuit whichoperates against a referenced thickness indicator. The referencethickness is a function of the particular location of thesource/detector combination along the path of travel of the substrate10. Thickness control circuitry 116 may equivalently include anappropriately programmed microprocesser; in either event, the thicknesscontrol circuit 116 derives signals for altering the RF power and/or thespeed of the web and/or the valves controlling gas flow upon detectionof a thickness which is not appropriate for the stage or location of thedetector 114. The control of the above-referenced variables is indicatedby the multiple outputs of the control circuitry 116: the power source100 (RF power control); drives of intermediate reels 20, 26 (web speedcontrol); and reaction gas processor 98 (valving).

The output of the detector 114 is additionally applied to a recorder 118which includes appropriate circuitry for decoding its output to producea histogram of thickness so that, in the event portions or segments ofthe substrate are unusable, they may be easily located, sectioned fromthe final product, and removed. If white light, as opposed to laserlight, is utilized in the thickness detection system, the opticaldetector and its associated control circuitry can be based upon theprinciple of thin film interference effects. When energy from a sourceof white light is reflected from a surface, such as the substrate 10,the thickness of the layer from which it is reflected may be determinedby comparing the intensity as a function of wavelength of the reflectedbeam with that of the incident beam. The equation for calculation of thethickness of the deposited film is: ##EQU1## where d=layer thickness

N=number of similar extrema of intensity displacing wavelength λ₁ fromλ₂

λ₁ λ₂ =wavelengths at chosen extrema of intensity

n₁ n₂ =index of refraction of layer material

θ=angle of incidence at wavelength λ₁ and λ₂

This equation applies rigorously at normal incidence.

A reference may be applied to the control circuit 116 corresponding to adesired thickness. Circuitry within the thickness control 116 convertsthe measured reflectance spectrum into a usable error signal which thecontrol 116 continuously drives toward zero by applying correctivesignals to any or all of the RF power source 110, the intermediate reeldrives 20 and 26 and the reaction gas processor 98. The details anddesign of servocontrol circuitry appropriate for generation of controlsignals in response to an error signal in the above manner is consideredconventional and well known in the electronics and electromechanicalarts both in terms of configuration and the elements required.

In the event that monochromatic light such as a laser, is employed, thethickness control circuit 116 is calibrated to count the number ofreflection extrema changed (peaks and valleys of intensity of reflectedlight). As is well known, the number of changes in intensity correspondsto a predetermined thickness. Therefore, each detector arrayed in thechamber should detect a different intensity of reflected light accordingto its location along the deposition "assembly line". The changes inintensity can thus act as the reference input for the control circuit116 in the same manner that the preselected thickness and pattern ofinterference colors provides a reference for the generation of an errorsignal when white light is employed. In either event, the circuit 116functions to provide appropriate control signals in response to theerror signals.

FIGS. 4A through 4F relate to the cathode 90 of the invention. Thecathode 90 serves the dual functions of (1) an electrode for the flowdischarge process and (2) a conduit for the flow of fresh reaction gasto and for the evacuation of the spent reaction gas from the plasmaregion to maintain a uniform, constant pressure glow discharge. Itincludes a top electrode plate 119 which is electrically connected tothe RF power source 100. A plurality of gas inlet openings 120 areuniformly arranged on the electrode plate 119. The openings 120 allowthe application of a uniform flow of fresh reaction gas to the surfaceof the substrate 10 (which serves as the anode of the glow dischargeprocess).

Uniformly spaced between the gas inlet openings are a plurality of exitvents 122 for collection of spent reaction gases. The vents 122communicate with the vacuum means of the reaction gas processor 98,providing a uniform removal of spent gases from the area between thesubstrate and the cathode. A screen 124, preferably of stainless steel,is associated with each of the exit vents, enhancing the uniformity ofexit gas flow by providing a plurality of uniformly-spaced openings atthe exit vent 122. The screen 124 is additionally useful for thecollection of solid contaminants.

The screen 124 is retained between a pair of stainless steel rings 126and 128. An insulating washer 130 surrounds the screen 124 and provideselectrical insulation between the rings 126 and 128. The screen 124 andthe lower ring 128 are grounded, as shown, to a lower baffle member,discussed below. The grounding of the screen isolates the pump outregion from the active plasma region between the electrode plate 119 ofthe cathode 90 and the advancing substrate 10. By grounding theseregions, spent reaction gases are evacuated through a dark space so thatgeneration of the plasma is confined to the region between the electrodeplate 119 and the substrate 10.

An insulator such as a sheet of ceramic paper insulation 134 is locatedadjacent the electrode plate 119. A plurality of openings 136 allows theflow of gas therethrough. The openings 136 are mated both with the gasinlet openings 120 and the exit vents 122 of the electrode plate 119.Gas distribution chambers 137 and 138 are located beneath the topelectrode plate 119. The chambers 137 and 138 are bounded and formed bytop, middle and lower baffle plates 140, 142 and 144, respectively. Thebaffle plates are illustrated in plan view in FIGS. 4C, 4D and 4Erespectively. By viewing the three plates simultaneously, one canobserve that each contains a plurality of holes 146 having cross-sectionequal to that of the exit vents 122 and arranged thereon so that, whenassembled, the holes 146 are aligned with the exit vents 122 of theelectrode plate 119. On the other hand, one may observe that thepluralities of holes 148 which serve as gas inlet openings are arrangedin various patterns throughout the three baffles 140, 142 and 144. Thepurpose of this deliberate misalignment from baffle to baffle is toassure that inlet gases applied to the chambers 136, 138 will travel anequal distance from a gas inlet 150 to the substrate 10 to equalize thereaction gas pressure across the surface of the substrate 10.

Reaction gases enter the system through the inlet 150 which communicateswith the reaction gas processor 98. A donut-shaped chamber 152 acceptsthe reaction gases and allows their eventual passage into the lower gasdistribution chamber 138 through the gas inlet holes 148 of the lowerbaffle plate 144. In such a manner, the reaction gases enter the lowergas distribution chamber, filling it until sufficient pressure causessubsequent flow (through the regularly spaced inlet holes of the middlebaffle plate 142) into the upper gas distribution chamber 136. The gasremains in the upper chamber 136 until sufficient uniform pressure onceagain results in its flow (through the holes of the top electrode plate118) into the area between the cathode 90 and the substrate 10. Theplasma is generated in and confined to this region by the interaction ofthe electrical field (imposed between the electrode 119 and thesubstrate 10) with the reaction gases.

A portion of the reaction gas is spent or unused during the glowdischarge decomposition process. Means are provided in the cathode 90for evacuating this spent gas in a uniform manner so that uniformity ofthe plasma chemistry over the entire surface area of the substrate ismaintained. Cylindrical, electrically insulative gas exit ports 154 ofceramic or the like extend the passageway for spent gases from theplasma area through the exit vents 122 and into a vacuum chamber 156.The ports 154 are aligned with the gas exit holes 146 of the baffleplates 140, 142 and 144. The chamber 156 communicates with the vacuum orpumping means of the reaction gas processor 98 through an exhaust port158. A baffle 160 is provided above the exhaust port 158 to equalize theflow paths of exhaust gases from the exit vents 122 to the exhaust port158 and to minimize any surface pressure differential resulting from theevacuation process. It has been found that, in utilizing a cathode 90according to the present invention, an extremely uniform layer ofamorphous material having uniform electrical properties is depositedupon the substrate. It is believed that the fact that all of thereaction gas must travel the same distance from the gas inlet 150 to thegas inlet openings 120 of the electrode plate 119 and that the spentreaction gas travels a substantially uniform distance from the plasmaarea to the exhaust port 158 assures a uniform pressure as well as auniform plasma chemistry at the surface of the substrate 10. It isfurther believed that this uniform pressure and plasma chemistrycontribute substantially to the uniform deposition of layers and to theuniform electrical properties such as photoconductivity and darkconductivity of the deposited layers. Such uniformity has not been foundto occur when reaction gas is applied to large area substrates by meansof laterally-spaced manifolds according to the prior art.

An alternative embodiment of the cathode is shown in cross-section inFIG. 4F. The cathode 162 differs from that of FIG. 4A by the addition ofa chamber 164 for spent gases. In addition, gas exit ports 166 are seento extend through the region formerly comprising the vacuum chamber 156.Equal flow paths may be obtained in this embodiment by the provision ofa plurality of baffles within the chamber having non-aligned holestherein to create subchambers in the manner of the gas distributionchambers 136 and 138 employed to equalize the flow paths of enteringreaction gas.

In operation, deposition occurs simultaneously in the chambers 28, 30and 32 in the plasma regions. In between the plasma regions of thechambers, adhering molecules of reactive gases are removed from thesubstrate 10 by means of the gas curtain associated with the opening inthe chamber separation valves. Once within a chamber, the electricallygrounded substrate 10 advances through the plasma region bordered by thesubstrate and the cathode of the chamber. The overall cathode mayconsist of a plurality of modules identical to those illustrated inFIGS. 4A through 4F to present a relatively large surface area in theaggregate. This relatively large area contributes to the uniformity ofthe layer deposited by reducing the edge effects that often complicatedeposition.

As the substrate 10 advances through the plasma region, a uniform flowof fresh reaction gas is directed at it through the top surface whichcomprises the electrode of the novel cathode. The reaction gas,including one or more elements or compounds of the alloy to be deposited(such as SiF₄) and one or more alterant elements (which may includesensitizers and dopants) is ionized by the field applied between thecathode and the substrate to form the plasma.

The rate of deposition of the semiconductor material from the plasmaonto the substrate 10 is a function of a number of variables, thecontrol of each of which is closely regulated throughout the system. Asmentioned, substrate temperature is maintained at between 250 and 300degrees Centigrade, while the monitoring of layer thickness serves tomonitor and control a number of process variables. In addition,applicants have found that a "low power," "low frequency" plasmagenerally contributes to depositing alloys of superior performancecharacteristics. While the technology of layer deposition from a plasmais new and not yet fully understood, it appears that a low kineticenergy of the depositing molecules which is attained in a low powerenvironment favors formation of a deposited structure having the leastamount of defects.

Applicants have developed a number of low power (requiring about 0.1watt/cm², as opposed to 1 watt/cm² for plasma generation) techniques foruse in a system according to the present invention. These techniquesgenerally result in a deposition which produces layers of improvedproperties and uniformity of thickness and chemical and structuralcomposition.

It is believed that resonance phenomena associated with high frequencyfields (such as the 13.56 MHz commonly employed for sputter and priorart glow discharge deposition) may induce oscillation of electronswithin the plasma. Such resonance may prevent the electrons fromtravelling across the gap between the cathode and the substrate toionize reaction gases in the vicinity of the substrate.

The desirability of low power, low frequency generation of thedeposition plasma has been empirically investigated by the Applicants.They have found that, when a plasma was generated at a frequency of13.56 MHz, significant plasma color nonuniformity was observed whichindicates a lack of uniformity of the chemistry within the plasmaregion. It is believed that nonuniformity of the plasma color is due tothe excitation of different species of the reaction gas at differentpoints. It was observed that at the higher frequency, 13.56 MHz, theplasma color at the position close to the gas inlet was blue whereas thecolor of the plasma close to the outlet was reddish. A higher depositionrate was observed in the blue region than in the reddish region. Incontrast, it has been found that a plasma formed of reaction gasinteracting with an electric field of 75 kHz possesses uniform color andhence deposits a layer of amorphous material of relatively uniformthickness. Layers of quite uniform composition can be obtained fromplasmas generated by electric fields in the frequency range of about 50kHz to about 200 kHz.

In addition to plasma and layer uniformity, it has been observed thatsilicon material deposited at 13.56 mHz is characterized by internaltensile stresses, while that deposited in the lower frequency rangeexhibits reduced internal stresses. Tensile stress is believed to causethe occasional peeling of deposited layers observed in the past. Thus,the structure and adhesion of the layers is enhanced by low frequency,low power deposition techniques.

Applicants have found that the deposition of intrinsic siliconexhibiting very favorable photoluminescence and photoconductivity isenhanced even at the relatively high frequency of 13.56 MHz when aquantity of inert gas such as Ar, Ne or He is introduced into themixture of reaction gas as a diluent. In particular, favorable resultswere observed when equal amounts of argon and a mixture of SiF₄ and H₂(in their customary ratio of 4-9:1) were combined. Here it is theorizedthat the energy profile of the plasma is significantly altered by theinclusion of the inert gas. The interaction of the depositing gas withthe various energy states of the ionized inert gas could easily enhancethe formation of plasma species favorable to the desired deposition.Thus, the inert gas acts as a kind of intermediate energy buffer toproduce a plasma with a profile of energies and species needed forefficient deposition of high quality amorphous Si alloys. (By the term"amorphous" is meant an alloy or material which has long range disorder,although it may have short or intermediate order or even contain attimes some crystalline inclusions).

Thus it is seen that there has been brought to the semiconductorprocessing art a valve especially adapted for isolating the glowdischarge/plasma deposition process within a chamber from undesiredreactive substances including, but not limited to, the atmosphere. Thevalve is of particular utility in the event a number of depositionchambers are serially arrayed along a device "assembly line" whereinvarious individual chambers are utilized to apply layers of variouschemical composition. The commingling of alterant elements in such eventmight render unusable the resultant device.

What is claimed is:
 1. A valve for use in a vapor deposition system (1)which is closed to atmospheric conditions and (2) wherein a substrate isadapted to sequentially travel through a plurality of operativelyinterconnected chambers for the successive deposition of semiconductorlayers thereonto; each chamber of said system including:spaced entranceand exit apertures through which said substrate travels; means formaintaining chamber pressures substantially below atmospheric pressure;means for introducing different reaction gas mixtures including at leastone semiconductor material into adjacent deposition chambers; and meansfor the vapor deposition of semiconductor material of differingconductivity types in each adjacent chamber; a valve adapted to extendbetween the entrance aperture of a succeeding chamber and the exitaperture of a preceding chamber for providing the operativeinterconnection between said adjacent deposition chambers: each valvehaving an opening communicating with the apertures of said adjacentdeposition chambers and through which said substrate material passes,the width of said opening being at least equal to the width of saidsubstrate material; means for introducing an inert gas alongsubstantially the entire width of said opening of said valve whereby aninert gas curtain is formed in the opening which substantially isolatesthe reaction gases in said adjacent deposition chambers and prevents thereaction gas mixture in one deposition chambers from contaminating thereaction gas mixture in the adjacent deposition chamber.
 2. A valve foruse in the system of claim 1, wherein said valve introducing andmaintaining means includes:a source of an inert gas; an inlet channelconnecting said source with said valve opening for introducing inert gasinto said valve opening; an outlet channel connecting said valve openingwith means for evacuating said inert gas from said valve opening; and agroove formed at the intersection of said channels and said valveopening, the length of said groove being substantially coextensive withthe width of said valve opening, whereby the inert gas is adapted toflow transverse to the direction of travel of said substrate material.3. A valve for use in the system of claim 2, wherein said inlet channelengages one end of said groove and said outlet channel engages the otherend of said groove;a pump adapted to introduce said inert gas into saidinlet channel; and said evacuating means adapted to regulate thepressure of said inert gas within said groove.
 4. A valve forsubstantially isolating the gas environments of adjacent, operativelyinterconnected deposition chambers which are (1) closed to theatmosphere, (2) maintained at subatmospheric pressures, and (3) adaptedto deposit successive semiconductor layers onto a substrate sequentiallypassing therethrough; said valve comprising:a valve opening throughwhich the substrate material passes between said adjacent chambers, theinterior of said valve opening being coated with a lubricant; a sourceof inert gas; means for introducing the inert gas into the valveopening; an inlet channel operatively connected at one end to said inertgas source and at the other end to said valve opening: means forevacuating said inert gas from said valve opening; an outlet channeloperatively connected at one end to said evacuating means and at theother end to said valve opening, whereby the introducing means and theevacuating means are operatively coordinated to maintain the pressure ofthe inert gas within the valve opening at a slightly greater level thanthe pressure of the gas environments in said adjacent depositionchambers to substantially prevent the flow of reaction gas betweenadjacent deposition chambers.
 5. A valve as in claim 4, including anelongated groove substantially coextensive and communicating with saidvalve opening; andsaid inlet and outlet channels operativelycommunicating with said valve opening via said elongated groove, wherebythe inert gas is adapted to flow transverse to the direction of travelof said substrate material.
 6. Deposition apparatus for continuouslydepositing semiconductor layers onto a substrate for the production ofphotovoltaic devices, the deposition apparatus comprising, incombination:at least two isolated deposition chambers into each of whicha different reaction gas mixture including at least one semiconductormaterial is introduced for depositing a successive semiconductor layeronto the substrate; means for the vapor deposition of semiconductormaterial of differing conductivity types in each adjacent chamber; and,an isolation valve operatively interconnecting each pair of adjacentdeposition chambers, the deposition chambers and the valve being closedto atmospheric conditions and adapted for operation at subatmosphericpressures; the valve including: an opening at least as wide as thesubstrate and through which the substrate continuously passes; a souceof inert gas; inlet means for connecting the source of inert gas withthe valve opening; means for introducing the inert gas into the valveopening; means for evacuating inert gas from the valve opening; outletmeans for connecting the evacuating means with the valve opening,whereby the introducing means and the evacuating means operativelycooperate to establish a flow of inert gases which substantiallyprevents contamination of the reaction gas mixture in one depositionchamber from the reaction gas mixture in the adjacent depositionchamber.